When the stimulable phosphor is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The stimulable phosphor then emits stimulated emission according to the level of the stored energy when the phosphor is exposed to electromagnetic wave such as visible light or infrared rays (i.e, stimulating light).
A radiation image recording and reproducing method utilizing the stimulable phosphor has been widely employed in practice. The method employs a radiation image storage panel comprising the stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the storage panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with a stimulating light to emit stimulated light; and photo-electrically detecting the emitted light to obtain electric signals giving a visible radiation image. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as stimulable phosphor sheet) has a basic structure comprising a substrate (i.e., support) and a stimulable phosphor layer provided thereon. However, if the stimulable phosphor layer is self-supporting, no substrate may be attached. The phosphor layer generally has a protective layer on its upper surface (surface not facing the substrate) to keep the phosphor layer from chemical denaturation and physical damage.
There are known a number of stimulable phosphor layers. A representative phosphor layer is formed by coating a dispersion of phosphor particles in a binder solution on the substrate and drying the coated dispersion on the substrate, and therefore comprises a binder and phosphor particles dispersed therein. Also known is a stimulable phosphor layer formed by vapor deposition and comprises prismatic phosphors in which a polymer binder is placed.
It is desired that radiation image storage panels have sensitivity as high as possible and further can give a reproduced radiation image of high quality (in regard of sharpness and graininess).
It is known that a radiation image storage panel have on a substrate a stimulable phosphor layer prepared by gas phase deposition such as vapor deposition or sputtering has a stimulable phosphor layer comprising multiple prismatic (or pillar-shaped) stimulable phosphor crystals standing on the substrate, and gives a reproduced radiation image with high sensitivity as well as high sharpness. This is because that the stimulable phosphor layer comprising multiple prismatic stimulable phosphor crystals has cracks between the adjoining prismatic phosphor crystals which favorably serve to efficiently receive a stimulating light and to efficiently release an emitted light and further serve to keep the stimulating light from scattering in a plane direction on the radiation image storage panel.
JP-A-2001-255610 describes a radiation image storage panel having a phosphor layer which is produced by gas phase deposition in which the phosphor layer has a thickness of 300 to 700 μm and a relative density (a ratio of the volume of the phosphor per the whole volume of the phosphor layer) in the range of 0.85 to 0.97.
JP-B-3070940 describes a method for depositing a stimulable phosphor layer on a substrate by decreasing a temperature of the substrate. It is stated that the method enables to produce a phosphor layer comprising prismatic phosphor crystals in which the prismatic phosphor crystals have no thick diameter even in the case that the thickness of the phosphor layer increases and the cracks are produced in the whole length in the depth direction. The resulting radiation image storage panel is stated to have an increased sharpness.
It is known that the phosphor layer in the radiation image storage panel which is produced by gas phase deposition should have a well-shaped prismatic structure extending in the depth direction to provide the phosphor layer with appropriate cracks, if a radiation image storage panel capable of providing a reproduced radiation image with a high sharpness is manufactured. If the volume of cracks is not enough, the reflection and scattering of the stimulating light as well as the emitted light decrease and the sharpness of the reproduced radiation image lowers. Particularly, in the case that the reproduction of the radiation image is carried out by line-detection, the sharpness of the reproduced image extremely lowers because of spreading of the emitted light. On the other hand, the phosphor layer should have a relative density as high as possible when it is desired to impart a higher sensitivity to a radiation image storage panel.
Nevertheless, a radiation image storage panel having a well balanced relationship between the image quality (such as sharpness) and the sensitivity is required particularly in the field of art of clinical test.